High band-gap opto-electronic device

ABSTRACT

A high band-gap opto-electronic device is formed by epitaxially growing the device section in a lattice-matched (Al x  Ga 1-x ) y  In 1-y  P-GaAs system. The band-gap of the epitaxial layer increases with x. Instead of growing the device section directly on the GaAs substrate, a layer of (Al x  Ga 1-x ) y  In 1-7  P, graded in x and in temperature while maintaining substantially y=0.5, is grown as a transitional layer. The high band-gap device structures include homojunctions, heterojunctions and particularly a separate confinement quantum well heterostructures. Various embodiments of the invention include devices on absorbing substrates and on transparent substrates, and devices incorporating strained-layer superlattices.

BACKGROUND OF THE INVENTION

This invention relates generally to opto-electronic semiconductordevices, and particularly, to a high band-gap quantum wellheterostructure laser and a method of growing high band-gapsemiconductor material.

An opto-electronic device serves to convert electric energy to lightenergy and vice-versa. It includes light emitting diodes (LEDs), laseremitters, photodetectors and photocells. In particular, LEDs arefrequently used for displays and as indicators, and laser diodes, whichmay be regarded as a form of LED, are used as fiber-optic sources.

Various types of semiconductor LEDs are known. In most types of LEDs, ap-n junction semiconductor is employed. A potential difference isapplied across the junction by means of a pair of electrodes in contactwith the p-type and n-type regions. This causes electrons to be injectedacross the junction from the n-type region to the p-type region andcauses holes to be injected across the junction from the p-type regionto the n-type region. In the p-type region, the injected electronsrecombine with the holes resulting in light emission; in the n-typeregion, the injected holes recombine with electrons resulting in lightemission. The wavelength of the light emission depends on the energygenerated by the recombination of electrons and holes which isdetermined by the band-gap of the p-n junction semiconductor material.

It is known in the art that the p-n junction may take on one of severalforms. In the simplest form, a homojunction device is employed, wherethe p-type and the n-type regions are of the same band-gap energy. Inimproved LEDs, a single heterojunction device is employed, where theband-gap energy in the p-type region is different from that in then-type region. This gives rise to the property that either electrons orholes, but not both, are injected across the junction. The injectedelectrons or holes then recombine to cause light emission in one regiononly. This region is commonly referred to as the active region. Byconcentrating the radiative recombinations in a smaller active region, aheterojunction device is more efficient than a homojunction device.

A device known as a double heterojunction LED further improves on theefficiency of single heterojunction LEDs. Typically, the active regionis sandwiched by a pair of wider band-gap layers, one being of p-typeand the other of n-type. Two heterojunctions are thus formed from thetriple layers. The higher band-gap of the additional layer helps toconfine the injected electrons within the smaller band-gap active layer.This allows for a much thinner active layer which minimizesre-absorption and increases light emission efficiency. Furthermore, thepair of higher band-gap layers also acts as cladding layers whichprovide optical confinement to further enhance light emissionefficiency.

In the case of laser emitters, a device known as quantum wellheterostructure (QWH) is highly efficient. A QWH may be regarded as adouble heterostructure where the thickness of the active layer isreduced to the order of carrier de Broglie wavelength. In this case, themotion of the carriers assumes a quantum effect and behaves like atwo-dimensional gas localized within the plane of the active layer. The2D quantization results in a series of discrete energy levels given bythe bound state energies of a finite square well. The correspondingdensity of states acquires a step-like function. In contrast, thedensity of states for the non-quantum counterpart is described by aparabolic function and diminishes to zero as the band edge isapproached. QWH are advantageous in that they have higher emissionefficiency, faster response time, lower threshold current and lowersensitivity to temperature variations.

The p- or n-type layers of various band-gaps are typically grown asepitaxial layers from the alloys of III-V compounds. One commoncompound, gallium arsenide (GaAs), readily yields high quality singlecrystals. However, it has a band-gap of 1.43 electron volts (eV) whichcorresponds to the infrared end of the light spectrum.

A wider band-gap material must be used to produce an LED with emissionin the visible spectrum. For example, efficient red LEDs have beenfabricated from aluminum gallium arsenide (AlGaAs) semiconductormaterial. AlGaAs is lattice-matched to GaAs. The band-gap energy ofsemiconductor material can be increased with substitution of aluminumatoms for gallium atoms. The greater the aluminum substitution in theresulting material, the higher is the band-gap. Aluminum is chosen toform the alloy because the varying concentration of aluminum does notsubstantially affect the lattice constant, and this property allowssuccessive epitaxial layers of lattice-matched AlGaAs to be growneasily.

Typically, to minimize re-absorption, the band-gaps of all layers arechosen to be wider than that of the active layer. In this way, theselayers appear transparent to the light emitted from the active layer. Bythe same consideration, the substrate on which the epitaxial layer isgrown should ideally have a wider band-gap. However, it is not possibleto obtain AlGaAs in wafer form, and instead, the lattice-matched,visible light absorbing GaAs is commonly used as a substrate.

The AlGaAs-GaAs system can at best provide red LEDs and lasers. Toobtain even shorter wavelength LEDs and lasers, such as in theorange-red or yellow part of the light spectrum, it is necessary toprovide semiconductor materials with still higher band-gap energieswhich are capable of epitaxial growth to form the various junctions. Tothis end, two classes of semiconductor alloy systems have been proposed:one is non-lattice-matched and the other is lattice-matched.

In a paper by Osbourn, Biefield and Gourley, published in AppliedPhysics Letters, Vol. 41, No. 2, July 1982, pp. 172-174, there isdisclosed a GaAs_(x) P_(1-x) -GaP system. This system is notlattice-matched, but the authors showed that layers can be grown withhigh crystalline quality if they are sufficiently thin strained-layersuperlattices (SLSs) and the composition of the layers is graded. Inthese structures, the lattice mismatch between layers is totallyaccommodated by strain in the layers, so that no misfit defects aregenerated at the interfaces. The authors fabricated an opto-electronicdevice which was shown by photoluminescence studies to have an emissionat a wavelength of 611 nm (corresponding to a band-gap of 2.03 eV) at atemperature of 78 K.

Since lattice matching is not required, SLSs can be grown from a widevariety of alloy systems and are consequently more flexible. Examples ofother lattice-mismatched materials include GaAs-GaAsP. However, nonlattice-matched systems cannot be grown as readily as lattice-matchedsystems, and their growth processes are generally more complex.

In the case of lattice-matched systems, it has been known that theAlGaInP-GaInP system can serve as the basis for growing higher band-gapdevices. The substitution of aluminum for gallium in Ga_(y) In_(1-y) Phas made possible the fabrication of high band-gap (Al_(x) Ga_(1-x))_(y)In_(1-y) P-(Al_(z) Ga_(1-z))_(y) In_(1-y) P heterojunctions and quantumwell heterostructures. Of these, the most important case is that of the(Al_(x) Ga_(1-x))₀.5 In₀.5 P alloy (y approximately equals 0.5), which(similar to Ga₀.5 In₀.5 P) is lattice-matched to GaAs and yields shorterwavelength lasers than the Al_(x) Ga_(1-x) As system. It has a largedirect band-gap up to 2.26 eV (549 nm), with potential for producing anemission wavelength in the range 555 nm to 670 nm at room temperature.

However, prior works with this system where active devices composed of(Al_(x) Ga_(1-x))₀.5 In₀.5 P were formed directly on GaAs substrateshave yielded devices with only moderately high band-gaps. Ishikawa etal, in Applied Physics Letters, no. 48, vol. 3, January 1986, pp.207-208, obtained continuous (cw) room-temperature (300 K.) laser diodeoperation at the red wavelength of 679 nm. Among the shortest wavelengthdevices produced thus far is one disclosed by Kawata et al, published inElectronics Letters, no. 24, vol. 23, November, 1987, pp. 1327-1328.They reported a (cw) room-temperature laser at 640 nm using an (Al_(x)Ga_(1-x))₀.5 In₀.5 P (x=0.15) active region in a double heterostructureconfiguration. As for pulsed, room-temperature operation, the shortestwavelength device is disclosed by Ikeda et al in Japanese Journal ofApplied Physics, vol. 26, 1987, pp. 101-105. They reported a pulsed,room-temperature laser at 636 nm using an (Al_(x) Ga_(1-x))₀.5 In₀.5 P(x=0.2) active region in a double heterostructure configuration.

The prior devices fall far short of realizing the full band-gappotential of the lattice-matched system described above. It is desirableto produce even shorter wavelength emitters by successfully growingpossible higher band-gap devices under the lattice-matched system.

SUMMARY OF THE INVENTION

One main reason prior devices have not achieved the higher band-gappossible is that unlike Al_(x) Ga_(1-x) As, (Al_(x) Ga_(1-x))_(y)In_(1-y) P is not intrinsically lattice-matched to GaAs. The indium (In)atom is larger than the arsenide (As) atom and the phosphor (P) atom issmaller. The lattice match with GaAs is delicately maintained bybalancing equal proportions of In and P atoms (i.e. y approximatelyequals 0.5). Prior devices have been forming the critical high band-gapdevice section (x approximately equals 0.4) directly on top of the GaAssubstrate. However, it is not a simple matter to keep the composition yfixed during the growth process, and therefore keep strain and defectsout of (Al_(x) Ga_(1-x))₀.5 In₀.5 P heterostructures grown on GaAssubstrates.

The present invention employs a simultaneous compositional andtemperature grading technique for growing epitaxial layers in alattice-matched alloy system by Metal Organic Vapor Phase Epitaxy(MOVPE).

An additional graded layer is grown to provide a relatively defect-freetransition from the substrate to the critical high band-gap devicesection. For example, in the case of (Al_(x) Ga_(1-x))₀.5 In₀.5 Pheterostructures on GaAs substrates, the graded layer is divided into anumber of sub-layers of different aluminum compositions and henceband-gaps. As each sub-layer is successively grown, the aluminumcomposition, x, increases from zero to a predetermined maximum value. Inthis way, the desired high band-gap, commensurate with that of thedevice section, is reached at the completion of the last sub-layer.

In addition, while the composition of the alloy is graded at eachsub-layer, the temperature is also graded. Thus, epitaxial growth foreach sub-layer is accomplished under the optimum temperature for itscomposition.

It is the simultaneous compositional grading and temperature increasethat allows for the successful growth of the remaining high band-gapdevice section.

Various high band-gap devices are realizable by the technique of theinvention. It includes homojunction, single heterojunction, doubleheterojunction devices, as well as quantum well heterostructures (QWHs).In the preferred embodiment of the invention, a high band-gap,separate-confinement QWH is formed as the device section.

QWH laser emitters produced by the techniques of the present inventionhave been reported by Kuo et al in a paper entitled `Stimulated Emissionin In₀.5 (Al_(x) Ga_(1-x))₀.5 P Quantum Well Heterostructure`, given atthe 4th International Conference on MOVPE, Hakone, Japan, May 16-20,1988. The paper reports lasing wavelengths as short as 543 nm (pulsed),553 nm (cw) at 77 degrees K., and 593 nm (pulsed), 625 nm (cw) at roomtemperature, 300 degrees K. These results represent the highest energylasers yet reported for the (Al_(x) Ga_(1-x))₀.5 In₀.5 P-GaAs system, orfor any III-V alloy system.

In one embodiment of the invention, a high band-gap device is formedwith an absorbing substrate. It includes a graded layer formed on a GaAssubstrate, followed by the device section.

In another embodiment of the invention, a high band-gap device is formedwith a transparent substrate. It includes a first part of a graded layerformed on a GaAs substrate, followed by a transparent substrate whoseband-gap is high enough not to absorb the emission from the activedevice, followed by a second graded layer to bring the band-gap to thelevel of the device section, followed by the device section itself. Theabsorbing GaAs substrate and the first part of the graded layer aresubsequently removed.

In yet another embodiment of the invention, a high band-gap device isalso formed with a transparent substrate. It includes a transparentsubstrate, whose band-gap is high enough not to absorb the emission fromthe active device, formed on a GaAs substrate, followed by a gradedlayer to bring the band-gap to the level of the device section, followedby the device section itself. The absorbing GaAs substrate issubsequently removed.

In still another embodiment of the invention, an inverted transparentsubstrate is formed with a high band-gap device. It includes a gradedlayer formed on a GaAs substrate, followed by the device section,followed by a transparent substrate whose band-gap is high enough not toabsorb the emission from the active device.

In still another embodiment of the invention, strained-layersuperlattice structures are incorporated into either the graded layer orthe device section or both.

Additional objects, features and advantages of the present inventionwill become apparent from the following description of a preferredembodiment thereof, which description should be taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-section of an active section grown on anabsorbing substrate, according to one embodiment of the presentinvention.

FIG. 1B is the band-gap energy diagram corresponding to the device ofFIG. 1A.

FIG. 2A shows schematically the details of the QWH residing in theactive section.

FIG. 2B is the band-gap energy diagram corresponding to the QWH of FIG.2A.

FIG. 3A is a schematic cross-section of an active section with atransparent substrate, according to one embodiment of the presentinvention.

FIG. 3B is the band-gap energy diagram corresponding to the device ofFIG. 3A.

FIG. 4A is a schematic cross-section of an active section with atransparent substrate according to another embodiment of the presentinvention.

FIG. 4B is the band-gap energy diagram corresponding to the device ofFIG. 4A.

FIG. 5A is a schematic cross-section of an active section with aninverted transparent substrate according to another embodiment of thepresent invention.

FIG. 5B is the band-gap energy diagram corresponding to the device ofFIG. 5A.

FIG. 6 is a schematic band-gap energy diagram corresponding to a devicewith SLS in the graded region and the cladding region.

FIG. 7 is a schematic band-gap energy diagram similar to that of FIG.1B, except the profile for the graded region is smooth.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A is a schematic cross-sectional view of an active, high band-gapsection grown on an absorbing substrate, according to one embodiment ofthe invention. Section 100 is a substrate of n-type or p-type, lowdislocation, single crystal gallium arsenide (GaAs), preferably ofthickness greater than 150 micrometers. A graded layer 130 of (Al_(x)Ga_(1-x))₀.5 In₀.5 P, having a thickness preferably in the range of 0.5to 1 micrometer, is formed over the GaAs substrate 100. The graded layer130 has a composition of (Al_(x) Ga_(1-x))₀.5 In₀.5 P where x variesover the thickness of the layer from 0 to 1.0. As more gallium atoms aresubstituted by aluminum atoms, the band-gap energy of the (Al_(x)Ga_(1-x))₀.5 In₀.5 P layer 130 increases. As a `primer` or buffer to thesubstrate 100, a 0.1 micrometer thin epitaxial layer of GaAs (not shown)may be optionally grown prior to the graded layer 130.

FIG. 1B illustrates the band-gap energy diagram corresponding to thedevice of FIG. 1A. Section 105 of the curve corresponds to the GaAssubstrate with a band-gap energy of approximately 1.43 eV. Section 135corresponds to the graded layer of (Al_(x) Ga_(1-x))₀.5 In₀.5 P.Initially, when the concentration of aluminum is zero, the band-gapenergy of Ga₀.5 In₀.5 P is approximately 1.9 eV as shown by the point130 on the energy curve. Towards the end of the growth, where 100% of Gaatoms are substituted by Al atoms, the band-gap energy of the gradedlayer has been raised to approximately 2.3 eV. This is illustrated bythe point 140 on the band-gap energy curve. Referring back to FIG. 1A,an active, high band-gap device section 150 is then formed on top of thegraded layer 130. Heterostructures of different band-gaps may be grownby varying the aluminum concentration, x, in (Al_(x) Ga_(1-x))₀.5 In₀.5P. After the device section 150 is grown, an optional contact layer 180having an energy level 185 may be grown on top of it. In this case, itis preferably an epitaxially grown GaAs or Ga₀.5 In₀.5 P layer ofthickness 0.1 to 0.2 micrometer. Finally, ohmic contacts may be formedon the device by conventional techniques.

In the process of growing the various layers, a horizontal Metal OrganicVapor Phase Epitaxy (MOVPE) reactor is used. The MOVPE reactor ismaintained at a low pressure of approximately 80 torr. The growth rateof the epitaxial layers is kept at about 0.04 micrometer per minute witha V/III ratio of 250. The epitaxial (Al_(x) Ga_(1-x))₀.5 In₀.5 P layersare grown lattice-matched (y approximately equals 0.5) on an n-type orp-type (100) GaAs substrate with trimethylaluminum (TMAl),trimethylgallium (TMGa), trimethylindium (TMIn), and 10% phosphine (PH₃)used as the sources for the primary crystal components Al, Ga, In, andP, respectively. Hydrogen selenide is used as n-type dopant source witha concentration between 5×10¹⁷ to 5×10¹⁸ atoms per cubic centimeter.Dimethylzinc is used as p-type dopant source with a concentrationbetween 5×10¹⁷ to 1×10¹⁸ atoms per cubic centimeter.

Two factors are found to be critical to high quality crystalline growth.One is to minimize oxygen and moisture contamination; the other is toobtain a high band-gap material by grading.

The aluminum contained in the AlGaInP alloy has a high affinity foroxygen. It has been found that the consequence of oxygen contaminationis heavy compensation and poor photoluminescence (PL) efficiency of thegrown layer. The problem is even more acute for those high band-gaplayers where the aluminum composition is high. Thus every precaution istaken to minimize oxygen and moisture contamination in the growthprocess. For example, the phosphine is passed through a molecular sievefilter to remove water vapor and oxygen. Reactants are carried bypurified hydrogen into the horizontal reactor tube containing anRF-heated graphite susceptor. The endcap to the reactor is enclosed by anitrogen-purged glove box to minimize oxygen contamination duringsubstrate loading and unloading.

The other factor critical to high quality crystal growth is to keepgrowth defects and strains out of the lattice-matched alloy. Accordingto the present invention, composition grading, previously employed onlyin non-lattice-matched systems, and temperature grading are used toproduce high band-gap devices using a lattice-matched system. Asmentioned in an earlier section, prior works on lattice-matched systemshave been to grow a high band-gap epitaxial layer on a GaAs substratedirectly in a single step. The present technique of composition gradingenables devices with even shorter band-gap (625 nm), previouslyunattainable, to be fabricated successfully.

To grow the graded layer of (Al_(x) Ga_(1-x))₀.5 In₀.5 P, such as thelayer 135 of FIG. 1A with its band-gap profile 135 in FIG. 1B, the layeris divided into a number of sub-layers. The first sub-layer has x equalto 0.0 and is composed of no aluminum, i.e. Ga₀.5 In₀.5 P, and may beregarded as a buffer layer between the GaAs and (Al_(x) Ga_(1-x))₀.5In₀.5 P layers. The last sub-layer has x approximately equal to 1.0,i.e., with complete substitution of aluminum. Each of the intermediatelayers has approximately equal increments in band-gap energy withcorresponding variation in aluminum concentration. In the limit ofdivision into many layers, the band-gap energy profile will be a smoothone (see FIG. 7.) In the preferred embodiment, approximately tensub-layers are successively grown and the band-gap energy profileassumes a step-like function.

In addition, while the composition of the alloy is graded at eachsub-layer, the temperature is also graded. Thus, epitaxial growth foreach sub-layer is accomplished under the optimum temperature for itscomposition. The optimum temperature may be determined experimentally byexamining the growth morphology of a sample of grown layers bytechniques such as x-ray diffraction. The optimum growth temperature forthe first sub-layer is found to be preferably about 690 degrees C.Similarly, the optimum growth temperature of the last sub-layer is foundto be approximately 755 degrees C. These two temperatures represent thelower and higher limits respectively. In one embodiment of theinvention, the optimum growth temperatures for the intermediatesub-layers may be estimated by linear interpolation of the lower andhigher limits.

Apart from the graded region, the epitaxial GaAs layers are preferablygrown at a temperature of approximately 690 degrees C. The remaininghigh band-gap device section is preferably grown at a temperature ofapproximately 755 degrees C. Indeed, it is the simultaneouscompositional and temperature grading that provides a relativelydefect-free transitional platform for the successful growth of thedevice section at a constant temperature of 755 degrees C.

While the description of growing the high band-gap layers has been givenin the context of the (Al_(x) Ga_(1-x))₀.5 In₀.5 P-GaAs system, it is tobe understood that the techniques of the present invention are equallyapplicable to other lattice-matched systems.

The high band-gap active section 150 may assume any of thehomostructural, heterostructural, double heterostructural andsuperlattice forms. In the preferred embodiment, a quantum wellheterostructure (QWH) is grown as the high band-gap active section 150.The corresponding band-gap energy diagram is shown in FIG. 1B as asection 155. FIG. 2A shows in more detail a cross-section of the QWH inthe active section 150 of FIG. 1A. Similarly, FIG. 2B is the detailedband-gap energy diagram corresponding to the QWH of FIG. 2A and to thesection 155 of FIG. 1B.

In particular, FIGS. 2A and 2B show a separate confinement QWH. Thisrefers to two separate sets of confining structures, one for confiningcarriers and the other for confining light.

The sections 161 and 162 form the high band-gap heterostructures oneither side of the active device 150. They help to confine the carrierswithin the central active layer. Section 161 is composed of (Al_(x)Ga_(1-x))₀.5 In₀.5 P where x is approximately 0.9. It is n-doped andpreferably grown to a thickness of approximately 1 micrometer. Thesection 162 is composed of p-doped (Al_(x) Ga_(1-x))₀.5 In₀.5 P where xis approximately 1. It is preferably grown to a thickness of 1micrometer. The corresponding band-gap energy diagram of the carrierconfinement layers 161 and 162 are shown in FIG. 2B respectively as thecurves 171 and 172. They correspond to a band-gap energy in the range of2.3 to 2.4 eV.

Referring back to FIG. 2A, the optical confinement layers are providedby layers 163 and 164. Both layers are composed of n-doped (Al_(x)Ga_(1-x))₀.5 In₀.5 P. They are both grown to a thickness ofapproximately 0.1 micrometer. Their corresponding band-gap energies areshown in FIG. 2B respectively as the levels 173 and 174 which correspondto 2.25 eV.

Finally, in FIG. 2A, the active quantum well section is comprised oflayer 167. The layer 167 is composed of n-doped (Al_(x) Ga_(1-x))₀.5In₀.5 P where x is equal to approximately 0.2. The active layer 167defines a quantum well with a width of approximately 0.02 to 0.04micrometers. The corresponding band-gap energy profile is shown in FIG.2B. The bottom of the well denoted by the level 177 is approximately 2eV, and corresponds to the layer 167 of FIG. 2A. In the fabrication ofthe QWH as illustrated in FIGS. 2A and 2B, each layer is grownsequentially from left to right.

Multi-quantum-well structures and other variations of the quantum wellstructure such as graded-index separate confinement heterostructures(GRIN-SCH) are well-known in the art. Although only single-quantum-wellstructures have been shown and described, the invention is equallyapplicable to multi-quantum-well structures and other variations such asGRIN-SCH.

Referring back to FIGS. 1A and 1B, the QWH device shown is typicallygrown on a GaAs substrate. As mentioned earlier, GaAs has a band-gapwhich corresponds to the infrared end of the light spectrum. It istherefore highly absorptive of, and opaque to, visible light. LEDdevices formed on GaAs substrates thus are inherently disadvantageous inthat the emitted light in the solid angles sustained by the opaquesubstrates, as well as light reflected by critical angle reflection atthe upper surface, is lost to absorption.

It is possible to have transparent substrate LEDs, an example of whichis disclosed by Ishiguro et al. in Applied Physics Letters, Vol. 43, No.11, pages 1034-1036, Dec. 1, 1983. The disclosed process involvesgrowing various transparent layers of AlGaAs on an absorbing GaAssubstrate. The first transparent layer adjacent to the GaAs substrateserves as a substitute substrate and subsequent layers constitute thedevice. The opaque GaAs substrate is subsequently removed, leaving thedevice on substitute transparent substrate only.

FIG. 3A is a schematic cross-sectional view of an active section with atransparent substrate, according to another embodiment of the invention.Essentially, the initial fabrication process is similar to that of theabsorbing substrate. The main difference is that during the growth ofthe grading layer one of the steps is very long in order to grow atransparent substrate.

Referring to FIGS. 3A and 3B at the same time, a GaAs substrate 300 withits corresponding band-gap energy level 305 may form the basis ofsubsequent epitaxial growth. As in the absorbing substrate casedescribed earlier, a grading layer 320 is epitaxially grown on top ofthe substrate 300. Because of the grading, the band-gap energy increasesas each new graded layer is grown, as shown by the energy level curve325. For a substrate to be transparent, its band-gap energy must behigher than the characteristic emission energy of the QWH. In thepresent case it is close to the level 377. When the band-gap energyreaches a level higher than the minimum band-gap energy level 377 of theQWH, the grading process is halted temporarily. Thereupon a layer 330 of(Al_(x) Ga_(1-x))₀.5 In₀.5 P of constant aluminum concentration (x=0.35)is grown. This layer is to be served as a transparent substrate and ispreferably grown to a thickness between 70 to 90 micrometers. Itsband-gap level is illustrated by the level 335. This level 335 may beanywhere between the levels 377 of the QWH and the band-gap energy level355 of the confinement layer 350. In the preferred embodiment, it ischosen to be slightly above the level 377 of the QWH so as to minimizethe aluminum content in the transparent substrate 335. In this way it isless susceptible to moisture and oxidation. After the transparentsubstrate level 330 is grown, a second graded layer 340 is grown on topof it to form a high band-gap basis (as shown by the energy level curve345) for the active section which follows.

The first and second graded layers 320 and 340 may be regarded asequivalent to the single graded layer 130 of FIG. 1A but split into twoparts. Indeed, if the growth of a transparent substrate such as thesubstrate 335 is included during the growth of graded layer 130 in FIG.1A, the resulting structures will be equivalent to the layers 320, 330and 340 of FIG. 3A.

Referring back to FIG. 3A and FIG. 3B, after the second graded layer 340is grown, an active, high band-gap section 350 is then grown on top ofit as in the case of the absorbing substrate illustrated in FIG. 1A.Finally a layer 380 acting as a protective cap as well as a contactbuffer is grown on top of the active section 350. It is most desirablefor the contact layer 380 to be transparent and therefore it should havea band-gap level 385 higher than the level 377. It is preferablycomposed of (Al_(x) Ga_(1-x))₀.5 In₀.5 P, where x is approximately 0.35.After all layers have been grown, any layers with band-gap lower thanthe level 377 of the QWH is removed. This prevents light emission fromthe QWH from being reabsorbed. In FIG. 3A, the absorbing layers to beremoved correspond to the GaAs substrate 300 and the first part of thegraded layer 320. The removal may be performed by chemical etching usinga solution such as a combination of hydrogen peroxide and sodiumhydroxide or hydrogen peroxide and ammonium hydroxide.

In order to achieve a good ohmic contact in some cases, the contactlayer 380 may be a thin absorbing layer of Ga₀.5 In₀.5 P (i.e. x=0).Once the metal contact is deposited (and it would not cover the entiresurface of the device) the Ga₀.5 In₀.5 P outside the area of the metalcontact would be etched away, leaving a mostly transparent region.

FIGS. 4A and 4B refer to another transparent substrate embodiment of theinvention. Again, as in the case illustrated by FIG. 3A, substrate 400(having energy level 405) may be a high quality GaAs wafer. However,unlike that of FIG. 3A, the first part of the graded layer is omitted,and a high band-gap transparent substrate 420 is grown directly on topof the substrate 400. Again the transparent substrate must have band-gapenergy level 425 higher than the band-gap energy level 477 of the QWHbut lower than the band-gap energy levels 455 of the confinement layer450 and 435 of the graded layer 430. The transparent substrate layer 420is composed of Al_(x) Ga_(1-x) As where x is approximately 0.75. Thesubstrate 420 is preferably grown to a thickness of 70 to 90micrometers. It could be grown epitaxially by liquid phase epitaxy (LPE)or metalorganic vapor phase epitaxy (MOVPE). Thereafter a graded layer430 followed by an active high band-gap section 450 and a protective capor contact buffer 480 (at energy level 485) are grown as in the caseillustrated in FIGS. 3A and 3B. After all the layers have been grown,the absorbing GaAs substrate 400 is then removed.

FIGS. 5A and 5B refer to yet another transparent substrate embodiment ofthe invention. In this case, the transparent substrate 560 is grownlast, particularly after the formation of the active section 550.Devices with such inverted substrate have been disclosed in a U.S.application Ser. No. 188,140 by Cook and Camras, assigned to the sameassignee of the present application. Their disclosure has been given inthe context of lower band-gap opto-electronic devices using the AlGaAssystem. The order of growing the transparent substrate and the activedevice layer is reversed. The active device layer is grown first on thehigh quality, but absorbing GaAs substrate. Then the transparentsubstrate is formed on top of the active device layer. Subsequently, theabsorbing GaAs substrate is removed. This enables the active devicelayer to be formed on a good surface, thereby ensuring consistently goodcrystalline quality, independent of any of the problems related to thegrowth of the transparent substrate.

Referring to FIGS. 5A and 5B, the growth of the absorbing substrate 500,the graded layers 530 and 540, and the active high band-gap section 550are all similar to the absorbing substrate embodiment described in FIGS.1A and 1B. After the formation of the active high band-gap section 550,a transparent substrate 560 is grown on top of it. The transparentsubstrate may either be Al_(x) Ga_(1-x) As where x is approximately 0.75or (Al_(x) Ga_(1-x))₀.5 In₀.5 P where x is approximately 0.6. It ispreferably grown to a thickness of 70 to 90 micrometers. Again as in thetransparent substrate described earlier, its composition is chosen suchthat its band-gap energy level 565 is larger than the level 577 of theQWH and less than the band-gap energy level 555 of the confinementlayers. Following growth and during processing, all layers which areabsorbing (i.e. having band-gap levels less than the level 577 of QWH)are removed. In the case of FIG. 5A, this corresponds to the originalGaAs substrate 500 and part of the graded layer 530 (corresponding toenergy levels 505 and 535).

FIG. 6 is similar to the schematic band-gap energy diagram of FIG. 1B,except with the incorporation of strained-layer superlattice (SLS) inthe graded region 635 and the confinement region 651 of the activesection. Lattice mismatch and strains are deliberately introduced intovery thin layers (with thickness approximately equal to 0.1 to 0.2micrometer) during the epitaxial growth of (Al_(x) Ga_(1-x))_(y) In_(y)P. The controlled introduction of strains into the epitaxial layer hasthe effect of accommodating the strains inherent in a material which isnot perfectly lattice-matched.

Referring to FIG. 6, to incorporate the SLS structures 635 and 651, theepitaxial growth of (Al_(x) Ga_(1-x))_(y) In_(y) P proceeds with averagegrowth parameters the same as in the case described in FIGS. 1A and 1B.In addition, y is allowed to fluctuate slightly (about 10%) about itslattice-matched value of 0.5 on alternate layers.

For illustrative purposes, FIG. 6 has SLS structures incorporated intoboth the graded region 635 and the cladding region 651 of the device onan absorbing substrate. It is to be understood that incorporation of SLSstructures is intended to further improve the crystalline quality whenthe need arises. It may be incorporated only in the graded region or inthe device section or in both regions (as illustrated) or not at all. Itis equally applicable to devices with transparent substrates.

Although the various aspects of the present invention have beendescribed with respect to its preferred embodiments, it will beunderstood that the invention is to be protected within the scope of theappended claims.

We claim:
 1. An electro-optical device comprising:a first substratelayer; a section composed of aluminum gallium indium phosphide havinghigher energy band gaps than the first substrate layer; and a secondlayer composed of aluminum gallium indium phosphide adjoining thesection so as to form a boundary therewith, said second layer having avarying energy band gap such that an energy band gap of said secondlayer at the boundary substantially equals that of the section at theboundary and decreases to a lesser energy band gap away from saidboundary, wherein said first substrate layer adjoins one of said sectionand said second layer and said first substrate layer, section and secondlayer are substantially lattice matched.
 2. The device of claim 1,wherein said second layer adjoins the section on one side and the firstsubstrate layer on the other.
 3. The device of claim 2, wherein thefirst substrate layer is composed of Gallium Arsenide.
 4. The device ofclaim 2, wherein the first substrate layer is a transparent substrate.5. The device of claim 4, wherein the first substrate layer is composedof aluminum gallium indium phosphide.
 6. The device of claim 4, whereinthe first substrate layer is composed of aluminum gallium arsenide. 7.The device of claim 1, wherein the section adjoins the second layer onone side and the first substrate layer on the other, said firstsubstrate layer being a transparent substrate.
 8. The device of claim 7,wherein the first substrate layer is composed of aluminum gallium indiumphosphide.
 9. The device of claim 7, wherein the first substrate layeris composed of aluminum gallium arsenide.
 10. The device of claim 1,wherein the energy band gap of the section is above 2 eV.
 11. The deviceof claim 1, wherein the energy band gap of the second layer changes bysteps.
 12. The device of claim 1, wherein the energy band gap of thesecond layer changes smoothly.
 13. The device of claim 1, wherein thefirst substrate layer and second layer have minimum energy band gapswhich are larger than that of the minimum energy band-gap of the sectionand less than the maximum energy band-gap of the section.
 14. Anelectro-optical device comprising:a first substrate layer; a sectionhaving higher energy band gaps than the first substrate layer andincluding a quantum well structure composed of doped semiconductormaterial to increase carrier density and light emission efficiency; anda second layer adjoining the section so as to form a boundary therewith,said second layer having a varying energy band gap such that an energyband gap of said second layer at the boundary substantially equals thatof the section at the boundary and decreases to a lesser energy band gapaway from said boundary, wherein said first substrate layer adjoins oneof said section and said second layer and said first substrate layer,section and second layer are substantially lattice matched.
 15. Anelectro-optical device comprising:a first substrate layer; a sectionhaving higher energy band gaps than the first substrate layer and astrain layer super lattice structure; and a second layer adjoining thesection so as to form a boundary therewith, said second layer having astrain layer super lattice structure and a varying energy band gap suchthat an energy band gap of said second layer at the boundarysubstantially equals that of the section at the boundary and decreasesto a lesser energy band gap away from said boundary, wherein said firstsubstrate layer adjoins one of said section and said second layer andsaid first substrate layer, section and second layer are substantiallylattice matched.